This invention relates generally to electronic instruments for servicing internal combustion engines and in particular to an instrument and method for determining relative compression between cylinders in an internal combustion engine by monitoring the battery voltage.
Internal combustion engines must have adequate compression levels within each cylinder in order to operate properly. Testing the compression level in each cylinder has traditionally been performed by removing the spark plug or diesel injector and measuring the pressure generated in the cylinder by turning the engine over through a cycle using an electric starter motor. The measurement taken thus provides an absolute compression level or a compression ratio. Measuring the pressure in this manner is tedious and time consuming, particularly when the goal is simply to determine whether or not the engine has a cylinder with a compression problem, and if so, to identify the cylinder or cylinders with compression problems so that they may be repaired.
A non-invasive technique of measuring relative compression has been successfully employed by automotive service technicians to diagnose compression problems. The relative compression test provides a visual comparison of all of the cylinders of the engine, typically in the form of a bar graph with the highest compression level determining the overall scaling factor. Relative compression testing is useful to the automotive service technician because in the typical engine service scenario because it provides a quick and reliable diagnosis of typical engine compression problems. Cylinders typically fail by losing compression rather than gaining compression. The remaining "good" cylinders may be used to determine an acceptable level of compression by setting an upper limit on the graph to compare the remaining cylinders to.
A basic measurement technique to perform a relative compression test has been detailed in U.S. Pat. No. 4,309,900, "Apparatus for Measurement and Display of Relative compression by Cylinder", to Kreft et al. Kreft et al. disclose an engine tester for determining relative compression by cylinder by sampling the starter current as timed by engine cylinder firings. A microprocessor correlates a cylinder number one signal with a predetermined firing order of the cylinders to display the relative compression level by cylinder.
Relative compression is typically measured by monitoring the battery current during the "cranking" phase when the electric starter motor turns the engine over through its cycle. One engine cycle is commonly understood to comprise a full 360 degree rotation of the engine as bounded by the successive firing of the cylinder number one. The engine is deliberately disabled so that it does not start during the cranking phase. The battery current varies as a function of the compression in each cylinder during the cranking phase because the starter motor draws more current as higher levels of compression create more mechanical resistance reflected back into the starter motor. The battery voltage in turn varies as a function of the battery current because the battery has internal resistance which develops a voltage drop. Either battery voltage or battery current may be measured to obtain a compression signal.
The advantage of the relative compression measurement technique over measuring the absolute pressure individually in each cylinder is the relative ease in obtaining useful diagnostic information about the compression levels in each cylinder engine. A handheld measurement instrument suitable for use in the automotive service environment to perform a relative compression measurement may be called upon for use in wide variety of automobiles. Furthermore, the electronic instrument may be used by automotive technicians who have only limited or infrequent experience in making a relative compression measurements. Thus, obtaining a relative compression measurement in a measurement instrument which achieves improved accuracy and with decreased susceptibility to sources of measurement error is desirable to the automotive service technician.